Brown sugar will get hard in a heart beat if you don't do something to keep the moisture in the container or bag. Here is a tip I use that really works and is pretty much free.
Take an old broken clay pot that you don't need anymore and break off a piece suitable for the size container that you keep your brown sugar. Make sure it is very clean and then soak it in water for about a half hour. Dry off with a towel and place in the container. This will keep your brown sugar nice and moist. Just repeat the soaking when it dries out, which should not be very often.
By littlegamma from southern, AL
I keep ours in the freezer.
The piece of clay pot is exactly what they make the cute little disc out off that you buy at any kitchen store for brown sugar softening. I actually got one at a yard sale, still in package. The instructions say "soak the disc in warm water for 15 minutes, insert into middle of brown sugar, it will keep moist for 3 months, then repeat the soaking." It does stay great for 3 months. To remember when to repeat I have placed a piece of masking tape on my sugar jar lid writing "re-soak disc in 3 months" then write the date the disc has been soaked. No need to wait until sugar gets hard when you have the info right there.